1. Field of the Invention
The present invention relates to an active noise control system for reducing noise generated in a duct for a fluid.
2. Description of the Related Art
Together with the increased density of today's residential and labor environments, cases in which noise sources such as air conditioning equipment and office equipment and residential spaces are in proximity to each other have increased. Up to the present, the muffling or the cut off of noise has been effectuated by making the distance between noise sources and people great. In addition, noise has been attenuated by installing a sound absorbing material. In such environments, however, it has been difficult to carry out above described measures.
Since noise in residential spaces is increased in the above manner, an amelioration of this situation is required. In addition, office equipment is provided with cooling fans and ducts for exhaust or for induction in order to cool down the heat emitting parts in the equipment. In this case, exhaust noise or induction noise for cooling is often annoying.
As for a general measure in order to reduce noise generated in a duct, there is a method for carrying out noise absorption processing by attaching a noise absorbing material on the inside wall of the duct. In addition, there is a method of reducing noise propagating through a duct by attaching a sound reducing muffler or a sound reducing chamber to an apparatus which emits a large amount of exhaust, such as an engine. There is a problem, however, that a large volume duct is required for reducing noise of a frequency of 1 kHz or less by means of these methods.
On the other hand, as for a method of reducing noise having low frequency bands without increasing the length or the volume of a duct, there is the proposal of introducing active noise control applied to an air conditioning duct. For example, there is the method as disclosed in Japanese unexamined patent publication S61-296392 (1986) or Japanese unexamined patent publication S62-1156 (1987). According to this method, a duct 1 is provided where a fluid A flows in the direction of Z and a noise B is propagated in the same direction as shown in FIG. 1. A noise detection microphone 2 is attached upstream in this duct 1 while a control sound source 4 and an error detection microphone 3 are attached downstream in this duct 1. Then, based on a reference signal from the noise detection microphone 2 and a residual signal from the error detection microphone 3, a control signal is generated by using an active noise control algorithm and a control sound is emitted from the control sound source 4 so that the residual signal becomes smaller.
In order to obtain a sufficient noise reduction effect by carrying out active noise control as described above, however, it is necessary that a sufficient coherence exists between the reference signal of the noise detection microphone 2 and the residual signal of the error detection microphone 3.
In addition, there is a method disclosed in Japanese unexamined patent publication S62-206212 (1987). According to this method, as shown in FIG. 2, a duct 5 is provided where a fluid A flows in the direction of Z and a noise B is propagated in the same direction. A first detection microphone 6 is attached upstream in this duct 5 while a second detection microphone 7 is attached at a distance b from the position of the first detection microphone 6, which is at a position downstream. A control sound source 8 is attached at a distance L (L>b) from the position of the first detection microphone 6, which is at a position downstream and which is outside of the duct 5. Then a signal from the first detection microphone 6 and a signal gained by carrying out delay processing on the second detection microphone 7 are synthesized so as to generate a control signal. Then, this control signal is given to a control sound source 8 and a control sound of which the phase is opposite to that of the noise is emitted and, whereby, noise control is carried out such that no howling is caused and that is in accordance with the propagation speed of the noise. In this method, it is also necessary that a sufficient coherence exists between the signal from the first detection microphone 6 and the signal from the second detection microphone 7.
FIG. 3 is a characteristics graph showing the relationship between the coherence γ between the noise detection microphone and the error detection microphone and an estimated reduction effect R which corresponds to the maximum noise amount reduced by active noise control. In the case that the coherence is 0.8 or more, the maximum noise reduction amount increase greatly. In order to obtain the sufficient noise reduction effect by means of active noise control as shown in FIG. 3, a high value of coherence is necessary. Due to the generation of disturbance, swirl or rotating flow within the duct, however, the coherence value between the two points is lowered. That is to say, the noise detection microphone and the error detection microphone detect not only a pressure fluctuation due to noise but also detect a pressure fluctuation due to disturbance, swirl, rotating flow or the like, so that the coherence value between the two microphones is lowered.
As for a method of solving this problem, a method of improving the coherence by rectifying the flow of fluid in a duct is proposed in Japanese unexamined patent publication H5-188976 (1993). For example, as shown in FIG. 4, a duct 9 is provided for expelling or for sending fluid A in the direction of Z. An air blower is provided upstream in this duct 9 and the case where this air blower functions as a noise source 10 is considered in the following. The fan of this air blower rotates, so that the fluid A and noise B flow in the direction of Z.
A noise detection microphone 11 is attached midstream within the duct 9 in the same manner as in the above described examples and a control sound source 12 and an error detection microphone 13 are attached, in this order, downstream within the duct 9. Then, an arithmetic circuit 14 is provided for generating a control signal based on a reference signal from the noise detection microphone 11 and a residual signal from the error detection microphone 13. In addition, a rectifying member 15A having a net form or a rectifying member 15B having a honeycomb form is inserted in the area downstream from the noise source 10 which is the area upstream to the noise detection microphone 11. Thus, air disturbance factors caused by the fan of the air blower are rectified in flow by rectifying member 15A or 15B. Thus, the coherence is improved between the different positions of the microphones downstream from the rectifying member.
In addition, there is a method disclosed in Japanese unexamined patent publication H9-89356 (1997). As shown in FIG. 5, a duct 16 in which fluid A and noise B are propagated in the direction of Z is provided. A noise detection microphone 17, a control sound source 18, an error detection microphone 19 and an arithmetic circuit 20 are provided in the same manner as in FIG. 4 and a metal net 21 is inserted in the area upstream to the noise detection microphone 17. The disturbance speed of the fluid A is attenuated by this metal net 21, so that an improvement in coherence is achieved.
In addition, in the case that the structure of the duct is complicated, there is a method of reducing noise of the fluid without using active noise control as described above by modifying the inside of the duct. As an example of this, the methods disclosed in Japanese unexamined patent publication H10-9877 (1998) and Japanese unexamined patent publication H10-39878 (1998) are shown in FIG. 6. Here, the case is considered: the bent portions of the duct 21 are formed of curved face walls 22a and 22b, so that the space surrounded by the curved face walls 22a and 22b is used as an air duct. In such a case, a rectifying plate 23 approximately parallel to the curved face walls is provided in the central part of the air duct. In addition, sound absorbing material is attached to the curved face walls 22a and 22b and to the surface of the rectifying plate 23. In such a structure, the disturbance factors or swirl factors of the air within the duct 21 are rectified when the air is expelled or transferred by an air blower 24. In this case, it is considered that the duct 21 itself has a rectifying part.
In active noise control systems provided for a duct in structures as described above, however, there are problem points as follows. That is to say, it is necessary in a variety of apparatuses that are equipped with active noise control systems, to further miniaturize air cooling ducts in order to achieve miniaturization of the apparatuses. In addition, there are cases where a plurality of heat emitting sources are provided in the apparatuses or air for cooling is supplied from a plurality of positions to the heat emitting sources. In these cases, it occurs necessity to bend the ducts, and to provide a plurality of ducts so as to merge them or to branch the ducts. In such cases, the forms of the ducts become complicated in comparison with the cases shown in FIGS. 1 to 5.
That is to say, in the case of ducts having a simple structure, air utilized for air conditioning can be rectified according to the above-described conventional measures and active noise control can be carried out by utilizing the coherence between the noise detection microphone and the error detection microphone, thereby obtaining the noise reduction effect. In the case that of ducts having complicated forms, however, sufficient active noise control cannot be carried out according to the above described conventional measures.